(feat) Added Configuration <NSCopying>

[robinmacharg]

Goal

Copy the passed BugsnagConfiguration object on BugsnagClient initialisation. This helps reduce unexpected mutation of a Client's configuration.

Design

Implemented <NSCopying> on BugsnagConfiguration, producing a shallow copy of itself. Updated config-dependent code to take copies appropriately.

Changeset

BugsnagConfiguration primarily with BugsnagClient making use of these.

Tests

Basic testing that copied configuration's properties don't mutate unexpectedly. Adjusted some existing tests that made assumptions based on the mutability of a passed-in config.

Review

Outstanding Questions

• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

• The correct target branch has been selected (master for fixes, next for
  features)
• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language